1. Field of the Invention
The present invention relates to fastening devices, and, more particularly, to fastening devices for fastening a composite laminate together.
2. Description of the Related Art
Mechanical properties of graphite composites make them attractive for structural applications, such as in aircraft and spacecraft, where high strength and stiffness-to-weight ratios are required. In structural applications, composite components are often fastened to other structural components (composites or metals) by mechanical means. In bolted composite structures, stress concentrations develop around the holes, severely reducing the strength of the structure. The regions containing holes must, therefore, be reinforced, resulting in an overall weight increase. Overdesigned joints can easily reduce the weight savings which are possible through the use of composite materials. In order to realize the full potential of laminated composite materials as structural elements, the strength and failure characteristics of mechanically fastened joints must, therefore, be optimized.
The strength and fatigue life of bolted joints in composite (and metal) structures are affected by a large number of factors. In particular, factors such as type of fastener, fastener/hole tolerance and lateral constraint affect the strength and fatigue life. In critical joints in thick composite structures, the non-uniform contact stress distribution through the thickness reduces the static strength and fatigue life of the structure significantly. For aerodynamical reasons countersunk fasteners are often required. The countersink reduces the static strength and fatigue life of bolted joints as compared with joints that have protruding head fasteners. The reason is due to several factors such as an unevenly distributed contact stress and low clamping force.
It is known for fasteners and fastener holes in aircraft structures to be cylindrical or cylindrical in combination with a conical countersink. Previously, it has been difficult in a production environment to machine a hole in an aircraft structure such that the hole has a complex geometry, i.e., such that an inner surface of the hole is curved or parabolic along its length. Because of this machining difficulty, complex fastener and hole geometries have not been previously used.
A bolted structure 10 (FIG. 1) is fastened by a known fastener 12 which may be protruding or countersunk. Fastener 12 is used to join plates 14 and transfer the load from one member to the other by shear forces in bolt 16. As the joint is loaded, bolt 16 bends and tilts in hole 18 causing a stress concentration through the thickness, which may significantly reduce the strength of the laminate. For optimum fatigue performance it is important to have an interference fit between fastener 12 and hole 18. A special problem when using interference fit fasteners in laminated composite materials is the risk for damaging the laminate when installing the fastener in the hole.
A severe disadvantage of existing aerospace fastener systems is the need for nuts or collars at the exit side of the hole or anchorage of the fastener. FIG. 3 shows a Hucktite type of lock bolt installed in a composite laminate. The parts of the fastener used for anchorage (shank and collar) represent a significant amount of the weight of the fastener. Moreover, the need for collars makes installation of blind rivets difficult in composite materials since the collar may cause damage to the composite material in the installation process. This is a well-known problem in the aircraft industry.
What is needed in the art is a fastening device and technique which reduces the non-uniform stress concentration through the thickness of a laminate to be fastened, and which makes it possible to install the fastening device with a well defined interference fit in both composites and metals without risking to damage the material.